1. Field of the Invention
The present invention relates to a television tuner which controls the oscillation frequency of a local oscillator by using a PLL circuit and can receive both a television signal and an FM broadcasting signal.
2. Related Art
A conventional television tuner comprises, as shown in FIG. 4, a VHF tuner 21, a UHF tuner 22, a PLL circuit 23, and a station selection control circuit 24. A VHF band television signal TV and an FM broadcasting signal FM are supplied to the VHF tuner 21 and a UHF band television signal TU is supplied to the UHF tuner 22. The VHF tuner 21 has an FM switching circuit 21a for performing switching between the VHF band television signal TV and the FM broadcasting signal FM and receiving the switched signal, a tuning circuit 21b, a mixer 21c, a local oscillator 21d, and the like. An intermediate frequency signal IF is outputted from the mixer 21c. 
The tuning circuit 21b can be switched between a case of receiving a television signal in the high band of the VHF band and a case of receiving a television signal in the low band of the VHF band. The FM broadcasting signal can be received in a state where the tuning circuit 21b is switched to receive the television signal in the low band of the VHF band. Although not illustrated, the UHF tuner 22 has also a tuning circuit, a mixer, and the like and an intermediate frequency signal of the UHF band television signal is outputted via the mixer 21c of the VHF tuner 21.
The PLL circuit 23 comprises a first fixed frequency divider 23a, a variable frequency divider 23b, a phase comparator 23c, a low pass filter 23d, a second fixed frequency divider 23e, and a reference oscillator 23f. The first fixed frequency divider 23a, the variable frequency divider 23b, the phase comparator 23c, and the low pass filter 23d construct a closed loop with the local oscillator 21d. The first fixed frequency divider 23a divides the frequency of a local oscillation signal outputted from the local oscillator 21d at a predetermined frequency dividing ratio. The variable frequency divider 23b further divides the frequency of the local oscillation signal divided by the fixed frequency divider 23a on the basis of frequency dividing ratio data DA outputted from the channel selection control circuit 24 in correspondence with the frequency of a reception channel and outputs the resultant signal to the phase comparator 23c. 
Meanwhile, a reference oscillation signal outputted from the reference oscillator 23f is also divided by the second fixed frequency divider 23e, thereby obtaining a reference frequency. The reference frequency is supplied to the phase comparator 23c. The phase comparator 23c compares the phases of two signals and outputs an error signal corresponding to the phase difference to the low pass filter 23d. The low pass filter 23d smoothes the error signal into a DC voltage. The DC voltage is used as a control voltage and applied to a varactor diode (not shown) in the local oscillator 21d, thereby controlling the frequency of the local oscillation signal outputted from the local oscillator 21d to be stable.
The station selection control circuit 24 receives a station selection signal S from a station selection operating unit in a television receiver (not shown), outputs the frequency dividing ratio data DA to the variable frequency divider 23b and outputs switching voltages BS1 to BS4 to the VHF tuner 21 and the UHF tuner 22. The switching voltage BS1 is outputted when the low-band television signal is received. The switching voltage BS2 is outputted when the high-band television signal is received. The switching voltage BS3 is outputted when the FM broadcasting signal is received. The switching voltage BS4 is outputted when the UHF band television signal is received. The switching voltages BS1 and BS2 are supplied to the tuning circuit 21b in the VHF tuner 21. The switching voltage BS3 is supplied to the FM switching circuit 21a in the VHF tuner 21. The switching voltage BS4 is supplied to the UHF tuner 22.
The tuning circuit 21b is switched so as to be tuned to the television signal in the low band of the VHF band and the FM broadcasting signal by the switching voltage BS1. The tuning circuit 21b is switched so as to be tuned to the television signal in the high band of the VHF band by the switching voltage BS2. The FM switching circuit 21a outputs the FM broadcasting signal to the tuning circuit 21b by the switching voltage BS3. Further, the switching voltage BS4 makes the UHF tuner 22 operative.
Each of the VHF television signal TV and the UHF television signal TU which are supplied to the television tuner has two kinds of television signals; what is called a ground wave television signal and a cable television signal sent via a cable (called a CATV signal). The ground wave television signal is according to what is called a standard frequency arrangement. The frequency of the CATV signal is, however, often slightly deviated from the frequency of the corresponding ground wave television signal. This is generally called an offset. The offset frequency lies within a range from few kHz to ten-odd kHz. The step frequency of the PLL circuit 23 is generally set to not 50 kHz but 31.25 kHz so as to be received even by an offset channel.
The step frequency indicates the minimum frequency interval of local oscillation signals outputted from the local oscillator 21d and is determined by the comparison frequency supplied to the phase comparator 23c and the frequency dividing ratio of the fixed frequency divider 23a. Generally, the oscillation frequency of the reference oscillator 23f is set to 4 MHz and the frequency dividing ratio of the second fixed frequency divider 23e is set to 1024, so that the reference frequency is 4000/1024=3.90625 kHz. In order to set the step frequency to 31.25 kHz, the frequency dividing ratio of the fixed frequency divider 23a is set to 31.25/3.90625=8.
The FM broadcasting signal is outputted at intervals of 100 kHz and, moreover, the frequency minimum unit is 100 kHz. When the step frequency of the PLL circuit 23 is 31.25 kHz, the oscillation frequency of the local oscillator 21d is therefore deviated from each of many FM broadcasting frequencies by ten-odd kHz. The sound quality of the FM broadcasting consequently deteriorates.
Accordingly, it is an object of the invention to provide a television tuner in which the step frequency of the PLL circuit 23 when the FM broadcasting signal is received is made different from that when the television signal is received, thereby realizing just-tuning of the FM broadcasting signal.
In order to solve the problem, there is provided a television tuner capable of receiving both of a television broadcast and an FM radio broadcast, comprising: a mixer for frequency converting a signal of each of the broadcasts into an intermediate frequency signal; a local oscillator for supplying a local oscillation signal to the mixer; and a PLL circuit for controlling the frequency of the local oscillation signal, wherein the PLL circuit varies the frequency of the local oscillation signal in accordance with the case of receiving the television broadcast and the case of receiving the FM radio broadcast by using different step frequencies.
Preferably, according to the television tuner, the PLL circuit comprises: a reference oscillator for outputting an oscillation signal of a predetermined frequency; a reference frequency divider for dividing the frequency of the oscillation signal and outputting the resultant signal as a reference oscillation signal; and a comparison frequency divider for dividing the frequency of the local oscillation signal outputted from the local oscillator and outputting the resultant signal as a comparison signal, the frequency dividing ratio of the reference frequency divider is switched between the case of receiving the television broadcast and the case of receiving the FM radio broadcast to thereby change the frequency dividing step frequency of the local oscillation signal in accordance with the time of receiving the television broadcast and the time of receiving the FM radio broadcast, and the frequency dividing ratio of the comparison frequency divider is switched, thereby changing the frequency of the local oscillation signal.
Preferably, according to the television tuner, the reference frequency divider has a frequency dividing ratio switching terminal and its frequency dividing ratio is switched when a reception switching voltage for switching a broadcast to be received from the television broadcast to the FM radio broadcast is applied to the frequency dividing ratio switching terminal.
Preferably, according to the television tuner, the frequency dividing step frequency of the local oscillation signal is set to 31.25 kHz in the event of receiving the television broadcast and to 50 kHz in the event of receiving the FM radio broadcast.